A METHOD OF SEPARATING METAL IONS FROM PULP

Abstract: A method of separating metal ions from pulp of lignocellulose-containing material. A main flow (1) of the pulp is treated with chelating agent (12) and washed (3) prior to the bleaching (5) of the pulp with hydroperoxide or ozone. Process water (4) washed out at the washing (3) and containing metal ions is mixed with pulp, which is not lateron to be bleached with hydroperoxide or ozone at pH above 7, whereby the metal ions substantially are bound to this pulp. Thereafter a liquid phase (9, 17) substantially free of metal ions is washed out of this pulp. Thereafter pH in this pulp is lowered to below 3 whereby the main part of the metal ions is released from the pulp and washed out.

Description: A METHOD OF SEPARATING METAL IONS FROM PULP This invention relates to a method of separating metal ions from pulp of lignocellulose-containing material in connection with the bleaching of the pulp with hydroperoxide or ozone, at which method a main flow of pulp is treated with chelating agent and washed prior to said bleaching. In pulp mills at present as well as in future there are and will be liquid flows, which are contaminated by various metal ions, which can disturb the bleaching reactions at bleaching with hydroperoxide or ozone. These liquid flows occur, for example at totally chlorine- -free bleaching, so-called TCF-bleaching (TCF = Totally Chlorine Free), when the oulp is treated with chelating agent, such as EDTA or DTPA in order to substantially reduce the metal ion content of the pulp prior to its bleaching with peroxide or ozone. This type of bleaching, in order to yield high ISO-brightness, requires both the pulp and the process water to be free or substantially free of certain metal ions, because otherwise the charged hydro¬ peroxide will effectively disintegrate to water and oxygen gas and, respectively, the charged ozone to oxygen gas, without having any simultaneous bleaching effect. Especially when the pulp mill shall be closed, for example for en¬ vironmental reasons, these liquids containing netal ions can be very difficult to deal with in order to prevent their contact with the bleaching, which would deteriorate the bleaching efficiency. The present invention offers a solution of the aforesaid problems. According to the invention, both the pulp and the process water are purified of metal ions in connection with the bleaching of the pulp with hydroperoxide or ozone. The characterizing features of the invention are apparent from the attached claims. The invention, thus, implies that undesired metal ions in concen¬ trated state are pushed out of the bleach plant, and that at the same time the bleached pulp is substantially free of metal ions, and process water separated from the pulp can be used as washing water without disturbing the bleaching process. A liquid flow containing the undesired metal ions in concentrated state is separated and can be treated separately, or be discharged to the recipient with¬ out causing serious damage on the environment. The metal ion cont¬ ent is as low as about 100 g manganese per ton pulp. τ hιs corres¬ ponds to the metal amount normally discharged from pulp mills producing TCF-pulp. The invention is described in greater detail in the following, with reference to the accompanying Figures 1-3 which by way of flow charts illustrate examples of different embodiments of the invention. _ At the embodiment according to Fig. 1, a main flow 1 of unbleached pulp is supplied to a pre-treatment step 2 where the pulp is treated with chelating agent of "Che type EDTA or DTPA. Metal ions, prefer¬ ably manganese, are here solved out of the pulp. In a subsequent first washing step 3, for example in the form of a washing press or washing filter, a liquid flow 4 containing the metal ions solved out of the pulp is separated. The main flow 1, from which the metal ions are removed, is directed to a bleaching step 5 for bleaching with hydroperoxide or ozone. After the bleaching, a partial flow 6 of the pulp JS separated. This partial flow 6 constitutes a small portion, suitably 5-20% and preferably about 10% of the main flow 1. To this partial flow 6 of the pulp, the liquid flow 4 containing metal ions is admixed, for example in a mixing device 7. By maint¬ aining pH above 7, suitably above 10, preferably at 11-12, the metal ions are bonded to the pulp. Thereafter the pulp in the part¬ ial flow 6 is washed in a second washing step 8, whereby the liquid phase 9 washed-out and free of the liquid can be re-used as washing water in the process. This liquid phase 9 will also contain the originally added chelating, agent, because it is not bound by the pulp, but follows along with the liquid. The partial flow 6 of the pulp which now contains bound metal ions is directed to a third washing step 10. Before washing, the pH is lowered to below 3, suitably to about 2,5, which can be brought about by the addition of sulphuric acid. The main part of the metal ions is hereby again released from the pulp and washed-out in the form of metal-containing waste water 11, which can be treated separ¬ ately or be emitted to the recipient. This waste water, totally seen, 3 corresponds to 1-2 m per ton pulp, counted on the main flow of pulp. A special advantage from an environment aspect is that the waste water 11 does not contain any chelating agent, because this follows along with the liquid phase 9. The main flow 1 of pulp is bleached with hydroperoxide or ozone in the bleaching step 5. After bleaching, the pulp is washed in a washing step 12, whereafter the partial flow 6 of pulp free of metal ions is reuinted with the main flew 1. The liquid phase 9 free of metal ions can be used as washing liquid in the washing step 12. The waste water 13 from this washing step 12 can be directed in a counter flow to the process, for example to a washing step 14 before the pre-treatment step 2. EXAMPLE The manganese content in a pulp delignified with xygen gas was meas¬ ured before and after the pre-treatment step 2, wherby pH was varied by sulphuric acid addition at the treatment. The pulp concentration was 5%, the temperature 90 and the treatment time 1 hour. The mang¬ anese content before the treatment was 61 g/ton pulp. After the treat¬ ment, the following values were measured. pH 2.6 3.5 7.2 9.6 H 2 S0 4 , kg/ton 10 5 2 0 Mn-content, g/ton 4,4 20. 45 48 This shows how the capability of the pulp to bind manganese ions varies with the pH-value. According to the invention, this relation was used to solve the problems with metal ions in connection with peroxide and ozone bleaching. The embodiment according to Fig. 2 agrees w th the embodiment shown in Fig. 1, with the exception that no partial flow is separated from the main flow 1 of pulp.A separate pulp flow 15 was used, inst¬ ead. This pulp flow 15 is directed in a separate circulation via the second and, respectively , third washing step 8 and, respectively, 10 whereby the pulp takes up the metal ions out of the liquid flow 4 and, respectively, emits the metal ions to the waste water 11. A further embodiment is shown in Fig. 3. There the main flow 1 of the pulp is used for separating metal ions. The liquid flow 4 separ¬ ated in the washing step 3 after the pre-treatment step 2 is re¬ cycled to the mam flow of pulp in the wasning step 16 after the bleaching step 5 The metal ions are here bonded tc the pulp in the way described above in connection with Fig. 1. The liquid phaεe 17 washed out and free of metal ions can thereby be re-rused as washing liquid jn the process, for example be returned to the washing step 14 before the pre-treatment step 2. The main flow 1 of pulp is thereafter passed through a further wash¬ ing step 18 where the metal ions are separated from the pulp in the way described above in connection with Fig. 1. The waste water 19 thereby washed out can be treated separately or be emitted to the recipient According to the lastmentioned embodiment, only one extra washing step 18 is required, compared with a conventional plant The metal ion concentration in the waste water 19, however, is lower than according to the embodiments 1 and 2. The invention, of course, is not restricted to the embodiments described and shown, but can be varied within the scope of the in¬ vention idea

Claims: Claims 1. A method of separating metal ions from pulp of lignocellulose- -contaimng material, in that a main flow (1) of the pulp is treated with chelating agent (2) and washed (5) prior to bleaching the pulp with hydroperoxide or ozone, c h a r a c t e r i z e d i n that process water (4) washed out at the washing (3) is mixed with pulp, which s not lateron to be bleached with hydroperoxide or ozone at pH above 7, whereby the metal ions are bound substantially to this pulp, that a liquid phase (9,17) substantially free of metal ions is washed out of this pulp, and that pH in this pulp thereafter is lowered to below 3 whereby the main part of the metal ions are released from the pulp and washed out. 2. A method as defined in claim 1, c h a r a c t e r i z e d l n that the pulp, which is not lateron to be bleached with hydroperoxide or ozone, consists of a portior (6) of the pulp ιr the man flow (1) , which partial flow (6) is separated after the peroxide or ozone-blearhing 3. A method as defined in claim 2, c h a r a c t e r i z e d l n that said partial flow (6) of pulp is recycled to the main fjow (1) after the metal ions had been washed out. 4. A method as defined in claim 1, c h a r a c t e r i z e d l n that the pulp, which is not lateron to be bleached with hydroperoxide or ozone, consists of the pulp in the main flow (1) after the peroxide or ozone-bleaching 5. A method as defined in claim 1, c h a r a c t e r i z e d l n that the pulp, which is not lateron to be bleached with hydroperoxide or ozone, consists of ,ι sepat jte pulp (15), which is circulated in a separate flow 6. A method as αefiπed in any one of the preceding claims, c h a r a c t e r i z e d i n that the separated liquid phase (9,17) substantially free of metal ions is re-used as washing liquid in the process

Inventors: GERMGAARD ULF, GERMGAARD, ULF
Applicants: GERMGAARD ULF, SUNDS DEFIBRATOR IND AB, GERMGAARD, ULF, SUNDS DEFIBRATOR INDUSTRIES AB
Application year: 1994
Main classification: D21C9
IPC codes: D21C9